Section 19. Every common carrier shall file with the department and shall plainly print and keep open to public inspection schedules showing all rates, joint rates, fares, telephone rentals, tolls, classifications and charges for any service, of every kind rendered or furnished, or to be rendered or furnished, by it within the commonwealth, and all conditions and limitations, rules and regulations and forms of contracts or agreements in any manner affecting the same, in such places, within such time, and in such form and with such detail as the department may order; provided, however, that such schedule shall not include a rate, fare, toll or charge for directory assistance service to the commonwealth or its political subdivisions and provided further that such schedules shall not include a rate, fare, toll, or charge to any customers based on the establishment and existence of the 508 area code. In the case of common carriers subject to the interstate commerce commission the forms prescribed for such schedules and the requirements relative to the filing and publication thereof shall conform, as nearly as may be, to the forms prescribed by and the similar requirements of the said commission. No common carrier shall, except as otherwise provided in this chapter, charge, demand, exact, receive or collect a different rate, joint rate, fare, telephone rental, toll or charge for any service rendered or furnished by it, or to be rendered or furnished, from that applicable to such service as specified in its schedule filed with the department and in effect at the time. Nor shall any common carrier refund, or remit directly or indirectly, any rate, joint rate, fare, telephone rental, toll or charge so specified, or any part thereof, nor extend to any person or corporation any rule, regulation, privilege or facility except such as are specified in the said schedule and regularly and uniformly extended to all persons and corporations under like circumstances for the like, or substantially similar, service. Unless the department otherwise orders, no change shall be made in any rate, joint rate, fare, telephone rental, toll, classification or charge, or in any rule or regulation or form of contract or agreement in any manner affecting the same as shown upon the schedules filed in accordance with this chapter, except after thirty days from the date of filing a statement with the department setting forth the changes proposed to be made in the schedule then in force and the time when such changes shall take effect, and such notice to the public as the department orders, to be given prior to the time fixed in such statement to the department for the changes to take effect. The department for good cause shown may allow changes before the expiration of said thirty days, under such conditions as it may prescribe, and may suspend the taking effect of changes under the circumstances and in the manner provided in the following section. At the time when any changes take effect they shall be plainly indicated upon existing schedules, or new schedules shall be printed and filed as the department may order. This section shall not prevent any telegraph or telephone corporation from continuing to furnish the use of its lines, equipment or service under any contracts in force on July first, nineteen hundred and thirteen, or upon the taking effect of any schedules of rates filed with the department as provided in this chapter, at the rates fixed in such contract; provided, that when any such contracts are or become terminable by notice, the department may direct by order that such contracts shall be terminated by the telegraph or telephone corporation which is a party thereto, and thereupon such contracts shall be terminated by such telegraph or telephone corporation as and when directed by such order. The department may exempt any common carrier from any provision of this section upon a determination by the department after notice and a hearing that such an exemption is in the public interest.